This invention relates to a device for detecting the unacceptable number of sheet-shaped materials piled one on another, for instance as in the double sheet detection in a sheet-fed press.
In general, an off-set press or the like employs a sheet-fed printing system in which sheets equal in size are fed to the printing machine one after another. Therefore, the printing machine often suffers from a so-called "double sheet" trouble that two sheets or more piled one on another are delivered to the printing machine at the same time.
If the double sheet trouble occurs, then blank sheets are mixed in the printed sheets. This will not only cause another trouble in the following process such as for instance bookbinding, but also damage the printing machine at worst. Accordingly, it is necessary to positively detect the presence of double sheets, thereby to prevent the delivery of two sheets or more piled to the printing machine.
Heretofore, a double sheet detecting method is usually employed, in which when a sheet supplied from a sheet supplying device is stopped by a stopper immediately before it goes into the printing process, the thickness of the sheet is measured, so that it is determined from the measured thickness whether or not the double sheet trouble occurs. In order to measure the sheet thickness, a mechanical method, as optical method or an electrical method has been employed.
In the typical example of the mechanical method, the sheet is depressed by a suitable contactor, so that the sheet thickness is measured from the displacement of the contactor. Accordingly, the mechanical method is disadvantageous in that it is difficult to measure the thickness of a sheet with high accuracy since a sheet is in general considerably thin, and accordingly the result of the measurement is not reliable. Furthermore, the surface of a sheet is liable to be damaged by the depression of the contactor, and whenever the kind of sheet is changed, a delicate adjustment is required.
In the optical method, light is applied to one side of a sheet, and the quantity of light passed through the sheet is measured to determine the thickness of the sheet. Accordingly, the optical method is advantageous in that no mechanical contact with the sheet is required, and therefore no damage is given to the sheet at all, and a thin sheet can be measured with high accuracy. However, the optical method is still disadvantageous in the following points: The measurement is liable to be erroneous for sheets such as hungry sheets which are not uniform in transmissivity. Furthermore, the optical method is not applicable to heavy sheets low in transmissivity and it is not suitable for the measurement of the thickness of a colored sheet other than a white sheet, because the thickness cannot be detected with sufficiently high accuracy. Especially in the case where both sides of a sheet are printed, in the second printing operation the portions of the sheet where patterns have been printed in the first printing operation cannot be used for the detection. Therefore, the detection is difficult, or impossible at worst.
In the typical example of the electrical method, the thickness of a sheet is detected from the variation of an electrostatic capacitance of a sheet to be measured which operates as the dielectric. The electrical method is advantageous in that, similarly as in the optical method, it is unnecessary to contact the sheet to be measured with a detecting element, and the measurement can be achieved irrespective of the transmissivity of the sheet; i.e. almost all the difficulties accompanying the optical method are eliminated. However, the electrical method still suffers from the problems that the measurement is liable to be affected by the drift of the electrical circuit and the variation in dielectric characteristic of a sheet due to the variations of the ambient temperature and humidity, and furthermore the measurement is affected by external electrical noise, as a result of which the accuracy of detection is not sufficient.